Flat panel display devices like Liquid Crystal Displays (LCDs) utilize flat glass sheets. A preferred technique for manufacturing these glass sheets is the fusion process. In the fusion process, the glass sheets are made by using vessels that contain refractory/precious metals, e.g. platinum or platinum alloys. The precious metals are generally considered to be inert in relation to most glasses, and thus should not cause any inclusions in the glass sheets.
However, this is not necessarily valid. There are oxidation reactions that occur at the metal/glass interface inside the vessels which lead to the generation of gaseous inclusions in the glass melt and thus the glass sheet. One of the more common oxidation reactions that occurs at the metal/glass interface is the conversion of negatively charged oxygen ions to molecular oxygen which is caused by the thermal breakdown of water and hydroxyl species in the glass melt. This phenomenon occurs because at the elevated temperatures of glass melting and delivery, a low partial pressure of hydrogen exists in the glass melt. And, when hydrogen comes in contact with the refractory/precious metal vessel containing the glass melt, the hydrogen rapidly permeates out of the vessel, depleting the metal/glass interface of hydrogen. Based on the chemical balance, for every mole of hydrogen that leaves the vessel, ½ mole of oxygen is left behind at the glass/metal interface. Thus, as hydrogen leaves the vessel, the oxygen level or partial pressure of oxygen at the metal/glass interface increases, which leads to the generation of blisters or gaseous inclusions in the glass melt. In addition, there are other reactions which involve the catalyzing or oxidation of other species in the glass melt such as halogens (Cl, F, Br) which can lead to the generation of gaseous inclusions. Further, the oxidation reactions can occur due to electrochemical reactions at the metal/glass interface. These electrochemical reactions can be associated with thermal cells, galvanic cells, high AC or DC current applications and grounding situations.
Today, there are several known methods that can be used to address these problematical oxidation reactions which cause the formation of gaseous inclusions in the glass sheet. One known method that can be used to help minimize the formation of gaseous inclusions in glass sheets involves the use of arsenic as a fining agent within the fusion process. Arsenic is among the highest temperature fining agents known, and, when added to the molten glass bath, it allows for O2 release from the glass melt at high melting temperatures (e.g., above 1450° C.). This high temperature O2 release, which aids in the removal of O2 bubbles during the melting and fining stages of glass production results in a glass sheet that is essentially free of gaseous inclusions. Furthermore, any residual oxygen bubbles are reabsorbed by the fining agent due to transition from the reduced to oxidized state on cooling. However, from an environmental point of view it is not desirable to use arsenic since it is considered a hazardous material.
There are several other known methods that do not need arsenic fining agents to mitigate oxidation reactions which lead to the formation of gaseous inclusions in the glass sheets. One such method is described in U.S. Pat. No. 5,785,726 which discloses a humidity controlled enclosure that surrounds one or more platinum-containing vessels and is used to control the partial pressure of hydrogen outside the vessel(s) so as to reduce the formation of gaseous inclusions in glass sheets. This humidity controlled enclosure is discussed in more detail below. Although the method disclosed in the patent mentioned above successfully reduces the formation of gaseous inclusions in the glass sheets, it would be desirable to provide an alternative method to prevent the formation of gaseous inclusions in glass sheets. This need and other needs are satisfied by the system and method of the present invention.